A method for producing a polyester fiber dyeable with basic dyes is known from Japanese Patent Publication No. 10497/1959 wherein the polyester fiber is produced from an aromatic polyester copolymerized with a compound having acid groups such as 5-sodium sulfoisophthalic acid. However, in this method, in order to satisfy the dyeability requirement, a large amount of acid component having the metal-sulfoisophthalic group hereinafter called ("S-Component") must be copolymerized. However the S component has the effect of increasing melt viscosity, so that the melt viscosity of a polyester containing a large amount of S-component and having a degree of polymerization sufficiently high to enable formation of a filament therefrom will become too high to enable the polyester to be spun in a conventional manner. Therefore, in order to spin the polyester containing a large amount of S-component in a conventional manner, it is usual that, in practice, its degree of polymerization is decreased in order to decrease the melt viscosity.
In this specification, the term "polymerization degree" is defined by the following equation: ##EQU1## wherein Ne=Number of end groups per 10.sup.6 g based on the polyester, and Wa=Average molecular weight of each repeating unit in polyester.
A polyester containing no S-component is hereinafter called a "conventional polyester" and that containing S-component is hereinafter called "S-containing copolyester".
For a given polymerization degree the tenacity of a polyester fiber containing a large amount of S-component is lower than that of the polyester fiber not containing S-component, so that usage of such an S-containing copolyester fiber is limited.
Furthermore, the hydrolysis rate of the ester of the S-component with glycol is higher than that of the ester of an acid component of a conventional polyester with glycol. Hence, when alkali scouring an S-containing copolyester fiber, those ester linkages which are between the S-component units and the glycol in the polyester fiber are hydrolysed selectively. This results in defects, in a direction perpendicular to the fiber axis, which occur to an extent greater than when scouring a conventional polyester. Therefore, when alkali scouring an S-containing copolyester fiber and a conventional polyester fiber of the same weight, the ratio of the tenacity of the alkali scoured polyester fiber to the tenacity of the polyester fiber which is not alkali scoured is higher for the S-containing copolyester fiber than for the conventional polyester fiber.
Even before scouring, the tenacity of an S-containing copolyester fiber is lower than that of a conventional polyester fiber. On scouring, this effect is multiplied by the even greater reduction in tenacity caused by scouring the S-containing copolyester fiber as compared with that caused by scouring the conventional polyester fiber.
This means that, in practice, alkali scouring (which is usually carried out on conventional polyester fibers in order to provide a fabric having good hand) cannot be carried out on fibers of a copolyester containing a large amount of S-component.
Furthermore S-component is expensive, so that copolyester fibers containing a large amount of S-component are also expensive.
Because of these drawbacks usage of copolyester fibers containing a large amount of S-component is limited.
In order to overcome these drawbacks, the amount of S-component copolymerized should be decreased. However, on decreasing the amount of S-component, the dyeability of the copolyester fiber will become low.
In order to allow a reduction in the amount of S-component and yet still achieve acceptable dyeability, it is known to include an additional acid component and/or to include a high molecular weight glycol component (hereinafter called a "G-component") in addition to the low molecular weight glycol component of a conventional polyester.
Japanese laid-open patent publication No. 158325/1970, mentions that polyester fiber formed from a copolyester containing isophthalic acid in an amount of 10 mol % based on the acid component and S-component in an amount of 2.5 mol % based on the acid component has a dyeability equal to that of a copolyester fiber containing 4.5 mol % of S-component based on the acid component. However, as mentioned in Flory, "Macromolecular Chemistry", the melting point of a copolyester containing 10 mol % isophthalic acid and 2.5 mol % S-component based on the acid component is remarkably lower than that of a conventional polyester.
If the melting point of the polyester is lowered, the temperature at which false twisting is carried out has to be lowered and therefore the crimp rigidity of false twisted yarn will be reduced and the hand of the fabrics formed from this false twisted yarn will become impaired.
As an alternative to the above copolyester, Japanese laid open patent publication No. 26006/1971 mentions that a copolyester fiber containing as copolymer methoxy-polyethylene glycol having a molecular weight of 2000 in an amount of 2 weight % based on the weight of the copolyester and containing copolymerized S-component in an amount of 2.5 mol % based on the acid component has a dyeability equal to that of a copolyester fiber containing 4.5 mol % of S-component based on the acid component. By that method a copolyester fiber which is excellent in dyeability and in crimp rigidity is obtained, but it appears that a copolyester fiber having high tenacity was not obtained. In that Japanese publication, it is mentioned that intrinsic viscosity of the polyester disclosed must be lower than that of conventional polyesters so as to obtain the particular characteristics needed for the invention described therein. However if a polyester has such a low intrinsic viscosity it must also have a low polymerization degree and therefore low tenacity.
Now, as mentioned in Flory, "Macromolecular Chemistry", as the amount of a comonomer in a copolymer is increased, and as the molecular weight of the comonomer is increased, so the melting point of the copolymer is reduced. When attempting to reduce the amount of S-component as described above, if a comonomer having large molecular weight but having a particularly high ability to improve dyeability is used, the melting point of copolyester will be reduced only slightly while retaining sufficient dyeability. Therefore, it is advantageous to employ a G-component having a large molecular weight as described in Japanese laid-open patent publication No. 26006/1971 mentioned above rather than to employ an additional acid component which is not an S-component as in Japanese laid-open patent publication No. 158325/1970 also mentioned above. This is because the ability of the G-component to compensate for loss of dyeability caused by reduction in the amount of S-component is very much higher than that of an additional acid comonomer. In addition, for the same degree of polymerization, the melt viscosity of a polyester containing both S-component and G-component is lower than that of a polyester containing only the same weight of S-component this rendering it easier to spin. This is because the G-component has the ability to lower melt viscosity.
On the other hand, the dyed fabrics of a copolyester fiber formed from a copolyester containing a large amount of G-component are unsatisfactory in light resistance. This contrasts with the disadvantage of using large amounts of S-component which provides copolyesters having high melt viscosity and which after spinning and drawing in the usual manner to form a yarn, provides fibers of low tenacity. Hence, to date, it was believed that a copolyester fiber containing S-component and G-component, having high tenacity and at the same time being excellent in dyeability, in alkali resistance and in light resistance of the dyed fabrics formed from the copolyester fiber is not obtainable.
Furthermore, it has been disclosed in Japanese patent Laid-open publication No. 107512/1980, that it is useful for achieving good depth of color to manufacture a polyester by incorporating 0.5 to 10 weight percent of inert inorganic microfine particles such as silica sol having an average diameter of no more than 100 millimicrons in a polyethylene terephthalate polymer, to melt-spin the polyethylene terephthalate polymer, to draw the resulting tow to obtain a polyester fiber and to extract the surface layer of the fiber with an alkali solvent and thereby cause an uneven dissolution of the inert microfine particles so that an irregular configuration with delicate projections and recesses is developed over the entire surface of the fiber.
However, such a process does not lead to sufficient improvement of the brightness and depth of color, particularly because it uses disperse dyestuffs. Therefore, it is preferable to obtain a sufficient effect of the brightness and depth of color by applying the abovementioned process to a cationic dyeable polyester. However, since both the presence of S-component and the inert inorganic microfine particles make viscosity high, it would be very difficult to melt spin such a polyester and indeed a cationic dyeable fiber which has both adequate depth of color and adequate tenacity for practical use after alkaline scouring treatment has not previously been obtained.